The state of the art includes applications wherein almost totally dry compressed air has to be available to avoid problems of corrosion and the formation of ice, both in distribution plants and in machines.
Among the drying plants most widely found in the state of the art are refrigeration drying plants, wherein the compressed air is cooled by means of a cooling system comprising at least a heat exchanger, in such a manner as to make the water contained in the air condense, and then separate it by separator means provided for this purpose and discharge it through suitable discharge devices.
At present two types of refrigeration drying plants are mainly used.
A first type, the so-called direct expansion type, uses at least a heat exchanger of the refrigerant/air type, connected to a refrigeration unit.
In this heat exchanger, the temperature of the compressed air, possibly already lowered by means of a pre-cooling heat exchanger of the air/air type located upstream, is lowered through direct exchange due to the contact between the walls defining the volume wherein the refrigerant circulates and the walls defining the volume wherein the compressed air to be dried circulates.
The air/air heat exchanger allows to carry out a partial energy recovery which by itself, however, does not allow to achieve any considerable energy saving or to increase to any large degree the overall thermal performance of the plant. In this type of plant, the cooling circuit is connected to the relative refrigerant/air exchanger by means of by pass valves or solenoid valves which allow to divide the flow of cooling fluid according to the delivery and/or temperature of the air to be processed.
This allows to prevent the water contained in the compressed air from freezing due to excessive cooling, but it does not ensure a proportional reduction of the overall energy consumption of the system.
Moreover, the cooling circuit is always switched on, irrespective of the fluctuating requirements of cooling capacity. This type of plant is therefore characterized by high efficiency because of the type of cooling (direct heat exchange between refrigerant and air) but has the disadvantage of high energy consumption because it can not reduce its capacity in direct proportion to a reduced load of the system.
A second type of refrigeration drying plant is the so-called (thermal mass) cycling type.
This type of plant has a configuration similar to that of direct expansion plants, with the difference that in the refrigerant/air heat exchanger the heat exchange does not occur directly but through a thermal mass, usually consisting of a liquid or a solid in particle form, located between the volume where the refrigerant circulates and the volume where the compressed air circulates.
In this second type of plant, the energy consumption is directly correlated to the volume and/or temperature of the air to be treated since, in the event that the volume of compressed air required by the user machines is reduced (and/or in the event of a lower temperature thereof), the excess cooling capacity produced is accumulated in the thermal mass.
In this way, the cooling circuit can be switched off for some periods of time, usually automatically according to the data monitored by a thermostat, and can continue to perform its cooling function as a result of the cooling capacity stored in and released by the thermal mass.
This possibility of using the cooling circuit cyclically allows to achieve a reasonable energy saving, proportionated to the load.
However, the quantity of latent cooling capacity accumulated by the thermal mass is not generally sufficient to ensure a high autonomy of the drying process when the refrigeration circuit is switched off, since during this stage the temperature of the thermal storage mass rises above the dew point temperature, so that the refrigeration circuit soon has to be re-started in order to ensure that the process continues.
In fact, the temperature of the thermal mass cannot be taken to very low values, for example less than zero .degree. C., because the thermal mass is directly in contact with the surfaces of the exchanger where the compressed air is flowing, and therefore a temperature of less than zero .degree. C. would lead to a risk of freezing the water contained in the compressed air.
This means that the cooling circuit has to be switched on and off with considerable frequency, which can cause a reduction in the working life of the drying plant, and can moreover reduce the energy saving which can be obtained with this type of plant.
Compared with the first type of direct exchange plant, plants of the thermal storage mass type, with thermal exchange of the indirect type, inevitably have a lower efficiency of heat exchange.
The present Applicant has designed and embodied this invention in order to supply a drying plant which will unite the advantages of the two types of conventional plants and eliminate the disadvantages in such a way as to ensure high performance and low energy consumption, and to obtain further advantages.